ABSTRACT ? OVERALL COMPONENT Neurodegenerative tauopathies, including Alzheimer's disease (AD) and frontotemporal lobar degeneration with tau inclusions (FTLD-tau), are characterized by tau inclusions of hyperphosphorylated tau. However, which tau species are pathogenic and how they induce neuronal dysfunction remain elusive. We hypothesize that there is a crosstalk between tau proteostasis imbalance, in the form of intracellular accumulation and intercellular spread, and neuronal dysfunction. Our vision statement is: Linking tau proteostasis with neuronal activity in FTD. We propose to use a combination of unbiased discovery and focused hypothesis- driven approaches to dissect the mechanistic connection. The Center, composed of three projects (P1?P3) and five cores (MS, CRISPR, Human, Data and Admin), aims at addressing the following fundamental questions in tau pathogenesis. In Specific Aim 1, we will address what causes tau to accumulate and spread in FTD. P1 and P3 will work together with MS core to dissect whether aberrant post-translational modifications (PTMs) are critically involved in modulating tau's failure to be degraded and subsequent release. P1, P2 and P3 will work together with CRISPR core to dissect the uptake and seeding mechanisms, using a combination of hypothesis-driven candidate approaches and genome-wide CRISPRi/a library screening. P1, P2 and P3 will work together with the Human core to validate the findings from iPSC neurons in human tissues. In Specific Aim 2, we will determine how tau proteostasis imbalance induces neuronal dysfunction. Despite large amount of evidence supporting the accumulation and spread of tau pathology in animal models and in cultured cells, little is known about the mechanisms behind this toxicity and their direct impact on neuronal/synaptic function in human neurons. P1 and P2 will work together to define the effects of FTD mutations on human neuronal activity both at the single-cell and network levels using whole-cell patch clamp and multi-electrode array (MEA), respectively. P3 will work with P1 and P2 to examine if altered autophagy affects neuronal activity. P1, P2 and the CRISPR core will work together to examine the functional effects of tau oligomerization. In Specific Aim 3, we will examine how neuronal activity modulates tau proteostasis. Both in vitro and in vivo studies showed that the release of tau is activity-dependent, supporting a feedback mechanism by which aberrant neuronal activity further alters tau proteostasis. P1 will work with the MS and CRISPR cores to dissect the mechanisms underlying the activity-dependent tau release. P1 and P2 will work synergistically to address if and how neuronal activity could affect the uptake (P2) or seeding (P1) of pathogenic tau. P3 will work with P1 and P2 to determine how altering neuronal activity modulates different autophagic pathways. In summary, the Center is strategically structured to maximize conceptual and technological synergies, full data integration and cross-validation. We anticipate discovery of novel insights into mechanisms underlying tau toxicity, generation of novel resources and reagents, and development of innovative technology platforms.